The present invention relates to forming and piercing tubular materials, and more particularly to cold forming and piercing tubular materials to produce structural members.
Tube hydroforming is a known method of cold forming metal tubes to create structural members, for example, for the automotive industry. In a typical hydroforming process, a tube is partially deformed by stamping it in a die. Then, internal hydraulic pressure exceeding the yield strength of the tube wall is applied to force the tube to expand and to conform to the die cavity--much like blowing up a balloon. Several references discuss hydroforming methods. These references include U.S. Pat. Nos. 5,339,667 issued Aug. 23, 1994 to Shah et al., entitled "Method for Pinch Free Tube Forming"; 5,070,717 issued Dec. 10, 1991 to Boyd et al., entitled "Method of Forming a Tubular Member with Flange"; and 4,744,237 issued May 17, 1988 to Cudini, entitled "Method of Forming Box-Like Frame Members"; and Sanjay Shah et al., Tube Hydroforming: Process Capability and Production Applications, Body Assembly & Manufacturing Proceedings, International Body Engineering Conference (September 1994).
Hydroforming processes offer several advantages over conventional die-stamping processes for cold forming metal tubes. These advantages include reduced variation in the finished pieces, reduced number of steps needed to produce the finished pieces, improved structural integrity of the finished pieces, and eliminated need to join separately pressed parts by welding. However, hydroforming has the disadvantage of requiring expensive and specialized die machinery to handle the extreme pressures to which the tube must be exposed. In particular, hydroforming requires additional machinery external to the die, such as pumps and intensifiers, to boost the internal hydraulic pressure of the tube. Further, the high pressures required for hydroforming can be dangerous to machine operators.
Several variations of the hydroforming process exist. For example, U.S. Pat. No. 4,829,803 issued May 16, 1989 to Cudini, entitled "Method of Forming Box-Like Frame Members" discloses a step of hydraulically pressurizing the internal space of a tube prior to closing the die, to allow better control of the deformation of the tube wall during die closure. The pressure to which the tube is initially pressurized, typically about 300 p.s.i.g., is selected to be less than the yield limit of the tube wall, but high enough so that during die closure (i.e., stamping), as the upper and lower die sections compress the tube, the tube walls are forced evenly toward the corners of the die cavity. More specifically, as the die closes, the hydraulic pressure within the tube causes the tube wall to overcome the frictional forces tending to resist the tube wall's transverse slippage over the surface of the upper and lower die sections. Thus, the internal pressure is selected so that the tube wall slides over the surface of the die sections and avoids being pinched between the upper and lower die sections as they mate.
To assure that the internal tube pressure during the '803 process does not rise to cause yielding of the tube wall during die stamping, a pressure relief valve is positioned in one end of the tube, set to release the liquid at a pressure below the yield limit of the tube. However, since the tube wall at the completion of this stamping process is bowed or dished inwardly, the '803 process requires a final hydroforming step of applying internal pressure to exceed the yield limit of the tube wall, and to expand the tube to conform to the die cavity. Thus, the '803 process does not escape the disadvantages of the hydroforming process. Rather, the '803 process adds an initial pressurization step to the hydroforming process, thereby slowing the tube forming process and increasing the cost of hydroforming.
Another variant of the tube hydroforming process is described in U.S. Pat. No. 5,353,618 issued Oct. 11, 1994 to Roper et al., entitled "Apparatus and Method for Forming a Tubular Frame Member," which discloses hydraulically pressurizing the interior of a tube to just below its burst pressure (yield strength) prior to bending and die stamping the tube, in order to ensure uniform, non-buckling deformation of the tube. A pressure relief valve and a hydraulic pressure source act in concert to maintain the internal pressure within the tube at just below the tube's burst pressure during the bending and stamping steps.
In utilizing the '618 process, if the cross-sectional perimeter of the preformed tube is, in some areas of the tube, less than the cross-sectional perimeter of the die cavity, then the tube must be expanded into the small radiused corners of the die cavity by subsequent hydroforming. However, if the cross-sectional perimeter of the die cavity is approximately equal to the cross-sectional perimeter of the preformed tube, then the tube will conform to the die cavity without subsequent hydroforming if the internal tube pressure prior to die stamping is near, yet less than, the internal burst pressure of the tube. (See Col. 18, Ins. 7-33.)
The '618 process has several disadvantages. The requirement that the tube be internally pressurized prior to die stamping adds a step that increases the complexity of the tube forming process, and increases the amount of equipment needed to complete the process. Further, subjecting the tube to high pressures prior to stamping requires a step that slows the forming process and therefore increases the cost of tube forming. Also, pressurizing a tube prior to stamping it decreases the safety of the stamping operation. A final disadvantage of the '618 process is the limitation that the internal pressure of the tube during the die stamping step remain below the yield strength (i.e., burst pressure) of the tube wall. This limitation ultimately requires an additional hydroforming step to expand the tube if the cross-sectional perimeter of the die cavity is greater than the preformed tube circumference.
It is known in the unrelated field of pipe bending that increasing the internal hydrostatic pressure of a pipe can help to prevent buckling or wrinkling of the pipe wall when bending the pipe. See, for example, U.S. Pat. Nos. 3,105,537 issued Oct. 1, 1963 to Foster, entitled "Bending Pipe"; 567,518 issued Sep. 8, 1896 to Simmons, entitled "Mechanism for Bending Pipe"; and 203,842 issued May 21, 1878 to Leland, entitled "Method of Bending Plumbers'Traps." The elevated hydrostatic pressures, although lower than hydroforming pressures, suffer the same disadvantages noted above. Further, the express purpose of pipe bending is to maintain the same cross-sectional roundness following bending.
It is also known in the unrelated field of bending pipes or tubes to fill the tube with liquid lead or lead-bismuth alloy and allow the metal to solidify prior to bending the tube in order to prevent the tube wall from buckling, collapsing, or wrinkling during the bending process. After bending, the metal-filled tube is heated to melt and drain the filling. See, for example, Bending Thin-Walled Tubing, Molings and Extruded Shapes published by Cerro Metal Products Company. Again, the express purpose of the tube-bending application is to prevent the distortion of the tube cross-section while bending the tube. Further, the use of a liquid metal fill when bending pipe or tubes presents the disadvantageous necessities: (1) cleaning and oiling the tube interior prior to filling the tube with the liquid metal, (2) cleaning the tube interior--frequently by chemical means--after draining the melted metal filling, (3) preventing the metal filling from oxidizing when melting it, and (4) preventing the metal filling from reacting with or sticking to the tube material. These additional steps are labor intensive and therefore expensive.
Holes are typically made in formed metal sheet by a punch process. To allow a clean pierce and prevent distortion of the metal area surrounding the hole, a "die button" is used to back up the metal sheet while the punch pierces the metal sheet. However, in formed, bent tubes, it is difficult to provide a back-up during the punch process. The geometry of the bent tube may prevent access to the tube interior in order to provide back up. For example, back-up of a punch operation is difficult if the formed tube has more than one bend along its axis or if the area to be punched is a substantial distance (e.g., more than about 12 inches) from the tube end. Further, bending a tube having holes can unacceptably distort the holes.
High-pressure liquid in the interior of a tube can provide support when piercing the tube. However, as with hydroforming, this method requires additional expense and equipment to boost the internal hydraulic pressure of the tube.
In the unrelated field of casting, it is known to prepare low-melting-point mandrels for use in the fabrication of high-melting-point structures. After casting, the cast unit is heated to melt and drain the mandrel. See, for example, U.S. Pat. No. 3,864,150 to Baird et al entitled "Reusable Mandrel for Structures Having Zero Draft or Re-Entrant Geometries" issued Feb. 4, 1975.